BoilerFaultCodesGlow-worm F77 Fault Code
The Glow-worm F77 fault code means: your boiler has detected an issue with the system that removes waste water or measured a temperature imbalance, causing it to shut down as a safety precaution.. It appears on 9 Glow-worm models in our database.
Technical description: Condensate pump / flue gas thermistor fault
What causes the Glow-worm F77 fault?
This fault occurs when the boiler detects a failure in the pump responsible for removing waste water (condensate) or a safety sensor indicates the internal temperatures are incorrect. The most common reason is a blocked or failed condensate pump, which prevents the boiler from safely draining liquid produced during operation. If the water cannot escape, or if a sensor detects abnormal heat levels, the system shuts down to prevent internal damage or flooding.
This fault is more frequent during winter, as the external drain pipe can freeze, causing water to back up and overwhelm the condensate pump.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Glow-worm models that show the F77 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Glow-worm F77 fault code information on this page is sourced and cross-checked against the official Glow-worm service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026